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June 05, 2007

A Sound Way to Turn Heat into Electricity

University of Utah physicists have developed small devices that turn heat into sound and then into electricity. The technology holds promise for changing waste heat into electricity, harnessing solar energy and cooling computers and radars.

"We are converting waste heat to electricity in an efficient, simple way by using sound," says Orest Symko, a University of Utah physics professor who leads the effort. “It is a new source of renewable energy from waste heat."

In the above photo, Symko demonstrates how heat can be converted into sound by using a blowtorch to heat a metallic screen inside a plastic tube, which then produces a loud tone, similar to when air is blown into a flute. Symko and his students are developing much smaller devices that not only convert heat to sound, but then use the sound to generate electricity.

Symko plans to test the devices within a year to produce electricity from waste heat at a military radar facility and at the university's hot-water-generating plant.

Symko expects the devices could be used within two years as an alternative to photovoltaic cells for converting sunlight into electricity. The heat engines also could be used to cool laptop and other computers that generate more heat as their electronics grow more complex. And Symko foresees using the devices to generate electricity from heat that now is released from nuclear power plant cooling towers.

Using sound to convert heat into electricity has two key steps. Symko and colleagues developed various new heat engines (technically called "thermoacoustic prime movers") to accomplish the first step: convert heat into sound.

Then they convert the sound into electricity using existing technology: "piezoelectric" devices that are squeezed in response to pressure, including sound waves, and change that pressure into electrical current. "Piezo" means pressure or squeezing.

Most of the heat-to-electricity acoustic devices built in Symko's laboratory are housed in cylinder-shaped "resonators" that fit in the palm of your hand. Each cylinder, or resonator, contains a "stack" of material with a large surface area – such as metal or plastic plates, or fibers made of glass, cotton or steel wool – placed between a cold heat exchanger and a hot heat exchanger.

When heat is applied – with matches, a blowtorch or a heating element – the heat builds to a threshold. Then the hot, moving air produces sound at a single frequency, similar to air blown into a flute.

Then the sound waves squeeze the piezoelectric device, producing an electrical voltage.

Devices that convert heat to sound and then to electricity lack moving parts, so such devices will require little maintenance and last a long time. They do not need to be built as precisely as, say, pistons in an engine, which loses efficiency as the pistons wear.

Five of Symko's doctoral students recently devised methods to improve the efficiency of acoustic heat-engine devices to turn heat into electricity. They will present their findings on Friday, June 8 during the annual meeting of the Acoustical Society of America at the Hilton Salt Lake City Center hotel.

The research is funded by the U.S. Army, which is interested in "taking care of waste heat from radar, and also producing a portable source of electrical energy which you can use in the battlefield to run electronics" he says.

The theoretical upper limit of conversion efficiency of heat to sound is the same as it is for conversion of heat to mechanical or electrical power:

Efficiency = (T hot - T cold)/T hot

There must be a heat source and a heat sink for conversion to take place. These are the T hot and T cold. Note that the temperatures are absolute temperatures (kelvins or degree Rankin).

To get kelvins, add 273 to °C. To get degrees Rankin, add 460 to °F.

So given a room temperature of 70°F (530°R) and a heat source 90° hotter at 160°F (620°R), the maximum amount of heat that could be turned into sound (mechanical energy, electrical energy) is (620 - 530)/620 = .145

That is, at most 14.5% of the heat could be converted. The other 85.5% of the heat is still heat exiting into the 70°F ambient.

Interesting idea. But one thing this story doesn't address is noise. I realize the point of the piezoelectric unit is to capture the sound waves, and therefore eliminate as much noise as possible. But isn't "leakage" inevitable? And if so, do you really want a noise generator to cool your computer or military-radar system?

Turning heat into electricity, especially from relatively small temperature differences, sounds worth pursuing but doing so cost effectively seems to be an ongoing problem. 90F degrees difference is still large and a lot of energy will go to waste. Using waste heat makes sense but we should be using those simple forms much more - heating water, heating and cooling buildings. Design, incorporating energy efficiency from the ground up could yield greater gains than thermo-acoustics.

I don't know what you are comparing that to, but power generation stirling engines typically use a temp difference of more than 500° F, and the solar towers that use steam turbines are way more than twice that.

This is the perfect energy scavenging tool. the waste heat coming out of a furnace or water heater stack or even a car exhaust pipe is a whole lot hotter than 160° F and when it is cold outside the greater temp difference makes even more energy available.

The big advantage of this tech is that it is cheap to make. A generator consists of a hollow tube with a ceramic grid that absorbs the heat, a moving coil on a diaphragm(like a loudspeaker coil) and a magnet, and that is just about it.

Even the materials are cheap. The tube could be made of almost any material: metal, concrete, conceivably even wood, and there is almost nothing to wear out, no cranks, no bearings, no complicated electronics.

What this tech lacks in efficiency it more than makes up in cheapness, reliability and simplicity, and with time the efficiency will improve.

As far as energy sources are concerned, an 8' parabolic dish can make a focal point hot enough to make a 2x4 burst into flame instantly (1500°+ F). Even if the efficiency of this gadget is only 15%, that is useful power, especially if you can heat your hot water with the 85% of the heat that is not being turned to electricity.

The article I posted on about this use for woodstoves mentioned that if the tubes were rigid enough the noise was limited to a slight hum, The more flex the tubes have the more they transmit the noise outward and away from the coil, creating quite a din and lowering efficiency.

The main article talks about rings(donuts) as one option where the noise does not reflect back from flat ends but continues around for mutltiple passes of the coil.

Without rigidity and insulation they apparently create well over 100 db.

Michael, I still think 90F is large, even if most heat engines use higher, but Stirling engines can and do run at lower differences, (according to Wikipedia, as low as 14F degrees), perhaps with lower efficiency. When it comes to waste heat, lots of it is less than 90F above ambient. Above that, if it's going to waste, it truly is wasteful. Still, if thermo-acoustics can find applications, or be significantly improved, thats only positive. In the meantime there is a lot of waste heat that doesn't need advanced technology to be put to use.

Some Stirling engines can run at even less than 6° F, but these engines have outputs in the low milliwatt (1000th of a watt) range, so they are essentially toys, or physics demonstrators. The problem with extracting energy from low temp differential sources is that there is not a lot of power there, or the power is so diffused that the mechanism to harvest it needs to be huge.

For a 90° temperature differential generator to be cost effective you would need a lake of it, and a cheap (non energy consuming) way to move it quickly past your engine.

Los Alamos has been working on this technology for several years. http://www.lanl.gov/mst/engine/
Ben and Jerry's ice cream also did a thermoacoustic refrigeration demonstration several years back.
I'm hoping that Symko isn't trying to take credit for "knovel" idea when it's already been reasonably well developed. Hopefully he finds a good application for an interesting physical phenomenon.

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Thermoelectric materials have long held the promise of being able to harvest waste heat and turn it into electrical energy. Although useful in applications other than energy generation, thermoelectrics have long suffered from poor efficiency,

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